The spatial Fourier transform technique is one of the well-known methods for measuring the angle-dependent reflection factor. Due to the effect of the finite measurement lines, using monopole sound sources leads to large deviations, especially at small angles of incidence. Thus, a sound source having proper directivity characteristics, such as a dipole source, is preferable. However, small deviations can still be found at small angles of incidence. In the present paper, a non-rectangular spatial window instead of the commonly used rectangular spatial window is applied to the sound pressures measured on two parallel lines, in order to decrease the effect of finite measurement lines. The performance of non-rectangular spatial window has been investigated by both simulation and experiment. It is shown that using the non-rectangular spatial window can improve the results of the method at small angles of incidence, especially for a monopole source. Therefore, more flexibility in the choice of the sound source is possible.